A stadium full of Yankees fans stood arm in arm at the bottom of the third inning in the Bronx, singing along to "Sweet Caroline," the Boston Red Sox anthem. Irony and sarcasm were absent. Sincerity was the mood of the night.

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The rival teams have buried the hatchet - at least for now. Yankee fans belted the Neil Diamond hit during a game Tuesday against the Arizona Diamondbacks, showing solidarity with their neighbor to the north a day after explosions at the Boston Marathon killed three people and injured more than 170.

"Everybody in New York knows what they're going through," said Mike Petti, 48, a Yankees fan nicknamed Yankee Mike who for 13 seasons has been a staple of the bleacher section where the most hardcore fans - those who hate Red Sox Nation the most - dwell. "When it happened here, Boston was singing 'New York, New York.'"

The teams' rivalry - which has reared its ugly head in bench-clearing brawls and fan assaults over the years - is just one difference between the cities, both among the oldest in the country and each with a rich history.

But look around New York this week, and you'll see nothing but love for Boston.

In what city officials said was a first, the bright-blue Boston city flag flew at half-staff at New York City Hall, on the orders of Mayor Michael Bloomberg, who grew up in the Boston suburb of Medford.

"We stand shoulder to shoulder with our friends in Boston, as all Americans do," said Bloomberg, who in 2006 founded the group Mayors Against Illegal Guns with Boston Mayor Thomas Menino. Bloomberg said he phoned his counterpart Monday to offer him any help New York could provide.

In Brooklyn on Monday night, an Occupy Wall Street group, the Illuminator, projected in large neon lights on the walls of the Brooklyn Academy of Music the two teams' logos, a heart inserted in between them, to read "NY(heart)B." The image has gone viral.

New York Roman Catholic Cardinal Timothy Dolan, celebrating Mass at St. Patrick's Cathedral in New York on Tuesday, said a special prayer for Boston.

Joe Daniels, president and CEO of the National September 11 Memorial and Museum, sent out an email offering comfort to Bostonians.

"The 9/11 Memorial is a constant reminder not only of what we have endured as a result of terrorism, but also of our ability to come together with limitless compassion," he wrote. "In the wake of the Boston attacks, this spirit of unity is more important than ever."

Tension has bubbled between Boston and New York since the 17th century, when the Puritans, who founded Boston, and the Dutch, who founded New York, squabbled over Long Island, said Professor Bill Fowler, a Northeastern University historian and former director of the Massachusetts Historical Society.

"We're so small compared to New York, but we're so powerful," said Fowler, acknowledging that Boston, with just over 600,000 residents, is smaller than the borough of Brooklyn. "On per capita basis, we've got you beat, it's just that you're bigger."

For Yankees fan Steve Sanzillo, who is still going back and forth with his wife, a Sox fan, over which baseball team their 19-month-old son Jackson will root for, Monday's attack did more to unify the cities than any rivalry could divide.

"It just brings back all the memories of 2001," he said. "I think I speak for all New Yorkers by saying our hearts are with Boston."

At Yankee Stadium, where police helicopters buzzed overhead and counterterrorism officers patrolled the outside, there was a sense of empathy among the fans, residents of a city no stranger to terrorism giving a knowing nod to their usually despised counterparts from Boston.

"I've been getting a lot of looks, kind nods," said Boston native Christine Sanzillo, 34, wearing in a Red Sox hat alongside her husband, a New York native. "It's a positive energy here today. Once my mother had a beer poured on her head."

During a moment of silence at the start of the game, a large commemorative ribbon with logos from the two teams was shown on the electronic board atop the ballpark.

"But what's going to happen when the Knicks and Celtics square off?" Petti - "Yankee Mike" - said, referring to the upcoming playoff game between the two cities' basketball teams.

"They'll show some respect," he said, "then they're going to want to beat each other."